Warfarin (WAR) is the primary oral anticoagulation drug in Saudi Arabia. Knowing the work of warfarin, its possible side effects, food and drugs interaction, warnings to be taken, and importance of adherence to warfarin have a significant role in accomplishing the needed therapeutic results while stopping the negative reactions. This study aims to assessment of knowledge and adherence on warfarin therapy among adult outpatients at king Fahd specialist hospital.